This invention relates generally to an amplifier and more particularly, to an operational amplifier suitable for use in an MIS integrated circuit constituted of MISFETs (insulated gate field effect transistors).
An amplifier having the circuit construction shown in FIG. 1 has been known in the past. In the drawing, reference numeral 1 denotes a differential input stage, 2 is an output amplification stage and 3 is a phase compensation circuit.
The differential input stage 1 consists of a pair of differential input MISFETs Q.sub.3 and Q.sub.4, load MISFETs Q.sub.1 and Q.sub.2 that are interposed between the drains of these MISFETs Q.sub.3, Q.sub.4 and a power source line DL and form a current mirror circuit, and a constant current MISFET Q.sub.5 interposed between the common source of the differential input MISFETs Q.sub.3 and Q.sub.4 and the reference potential line SL of the circuit.
The output amplification stage 2 consists of an MISFET Q.sub.6 interposed between the power source line DL and an output terminal V.sub.out and a constant current MISFET Q.sub.7 interposed between the output terminal V.sub.out and the reference potential line SL of the circuit.
The phase compensation circuit 3 consists of a resistor R.sub.1 and a capacitor C.sub.5 that are connected in series between the output terminal V.sub.out and the drain of MISFET Q.sub.2 described above.
A reference voltage V.sub.ref1 is applied to the gates of MISFETs Q.sub.5 and Q.sub.7 so that these transistors Q.sub.5 and Q.sub.7 operate as constant current sources.
MISFETs Q.sub.1, Q.sub.2 and Q.sub.6 are p-channel MISFETs while MISFETs Q.sub.3, Q.sub.4, Q.sub.5 and Q.sub.7 are n-channel MISFETs.
The amplifier described above can constitute a voltage follower by connecting its inversion input terminal IN.sub.1 to its output terminal V.sub.out so as to have its non-inversion input terminal IN.sub.2 function as the input terminal.
If this input terminal IN.sub.2 is connected to a ground potential point, the voltage follower is believed to be substantially insensitive to the power source noise. In other words, when the potential of the power source line DL moves up and down due to the power source noise, the potential at a node n.sub.1 and the source potential of MISFET Q.sub.6 also move up and down in response to the former. Accordingly, transient noise appears at the output terminal V.sub.out. Since the output terminal V.sub.out is connected to the inversion input terminal IN.sub.1, that is, since feedback is applied, however, it is generally believed that virtually no noise appears at the output terminal V.sub.out.
As a result of examination of the voltage follower composed of the amplifier described above, however, the inventors of the present invention have found that noise does appear at the output terminal V.sub.out when noise of a relatively high frequency is applied to the power source voltage.
In other words, since the phase compensation circuit 3 or the like is disposed at the drain of MISFET Q.sub.2, the value of capacitance coupled to this transistor Q.sub.2 and the value of capacitance coupled to the drain of MISFET Q.sub.1 do not balance with each other, so that a phase difference occurs between the noise appearing at the drain of MISFET Q.sub.2 and the noise appearing at the drain of MISFET Q.sub.1.
In the case of noise of a low frequency, the potential difference between the drain of MISFET Q.sub.1 and that of MISFET Q.sub.2 that occurs due to this phase difference is relatively small. For this reason, the drain voltage of MISFET Q.sub.1 is substantially changed in such a direction as to restrict the potential change of the output terminal V.sub.out by feedback and hence, the potential change of the output terminal V.sub.out is small in the case of power source noise of a low frequency.
In contrast, in the case of a power source noise of a relatively high frequency, e.g., around 300 KHz, the potential difference between the drain of MISFET Q.sub.1 and that of MISFET Q.sub.2 occurring due to the phase difference is relatively great, so that the drain voltage of MISFET Q.sub.1 is changed in such a direction as to enlarge the potential change of the output terminal V.sub.out by applying feedback. Moreover, the capacitor C.sub.5 forming the phase compensation circuit cannot cut the power source noise of a relatively high frequency but transmits it as such to the output terminal V.sub.out. For these reasons, the potential change at the output terminal V.sub.out reaches a level that cannot be substantially neglected, in the case of power source noise of a relatively high frequency.